Compile WebAssembly to JVM and other WASM tools

Asmble

Asmble is a compiler that compiles WebAssembly code to JVM bytecode. It also contains
an interpreter and utilities for working with WASM code from the command line and from JVM languages.

Features

WASM to JVM bytecode compiler (no runtime required)

WASM interpreter (instruction-at-a-time steppable)

Conversion utilities between WASM binary, WASM text, and WASM AST

Programmatic JVM library for all of the above (written in Kotlin)

Examples showing how to use other languages on the JVM via WASM (e.g. Rust)

Quick Start

WebAssembly by itself does not have routines for printing to stdout or any external platform features. For this
example we’ll use the test harness used by the spec. Java 8 must be installed.

Download the latest TAR/ZIP from the releases area and extract it to
asmble/.

WebAssembly code is either in a binary file (i.e. .wasm files) or a
text file (i.e. .wast files). The following code imports the print
function from the test harness. Then it creates a function calling print for the integer 70 and sets it to be called
on module init:

Which is how the test harness prints an integer. See the examples directory for more examples.

CLI Usage

Assuming Java 8 is installed, download the latest release and extract it.
The asmble command is present in the asmble/bin folder. There are multiple commands in Asmble that can be seen by
executing asmble with no commands:

Usage:
COMMAND options...
Commands:
compile - Compile WebAssembly to class file
help - Show command help
invoke - Invoke WebAssembly function
run - Run WebAssembly script commands
translate - Translate WebAssembly from one form to another
For detailed command info, use:
help COMMAND

Note, that any Java class can be registered for the most part. It just needs to have a no-arg consstructor and any
referenced functions need to be public, non-static, and with return/param types of only int, long, float, or double.

Asmble can translate .wasm files to .wast or vice versa. It can also translate .wast to .wast which has value
because it resolves all names and creates a more raw yet deterministic and sometimes more readable .wast. Technically,
it can translate .wasm to .wasm but there is no real benefit.

All Asmble is doing internally here is converting to a common AST regardless of input then writing it out in the desired
output.

Programmatic Usage

Asmble is written in Kotlin but since Kotlin is a thin layer over traditional Java, it can be used quite easily in all
JVM languages.

Getting

The compiler and annotations are deployed to Maven Central. The compiler is written in Kotlin and can be added as a
Gradle dependency with:

compile 'com.github.cretz.asmble:asmble-compiler:0.3.0'

This is only needed to compile of course, the compiled code has no runtime requirement. The compiled code does include
some annotations (but in Java its ok to have annotations that are not found). If you do want to reflect the annotations,
the annotation library can be added as a Gradle dependency with:

compile 'com.github.cretz.asmble:asmble-annotations:0.3.0'

Building and Testing

To manually build, clone the repository:

git clone --recursive https://github.com/cretz/asmble

The reason we use recursive is to clone the spec submodule we have embedded at src/test/resources/spec. Unlike many
Gradle projects, this project chooses not to embed the Gradle runtime library in the repository. To assemble the entire
project with Gradle installed and on the PATH (tested with 4.6), run:

gradle :compiler:assembleDist

Library Notes

The API documentation is not yet available at this early stage. But as an overview, here are some useful classes and
packages:

asmble.ast.Node - All WebAssembly AST nodes as static inner classes.

asmble.cli - All code for the CLI.

asmble.compile.jvm.AstToAsm - Entry point to go from AST module to ASM ClassNode.

asmble.compile.jvm.Mem - Interface that can be implemented to change how memory is handled. Right now
ByteBufferMem in the same package is the only implementation and it emits ByteBuffer.

FuncBuilder - Where the bulk of the WASM-instruction-to-JVM-instruction translation happens.

asmble.run.jvm - Tools for running WASM code on the JVM. Specifically ScriptContext which helps with linking.

asmble.run.jvm.interpret - The interpreter that can run WASM all at once or allow it to be stepped one instruction
at a time.

Note, some code is not complete yet (e.g. a linker and javax.script support) but beginnings of the code still appear
in the repository.

And for those reading code, here are some interesting algorithms:

asmble.compile.jvm.RuntimeHelpers#bootstrapIndirect (in Java, not Kotlin) - Manipulating arguments to essentially
chain MethodHandle calls for an invokedynamic bootstrap. This is actually taken from the compiled Java class and
injected as a synthetic method of the module class if needed.

asmble.compile.jvm.msplit (in Java, not Kotlin) - A rudimentary JVM method bytecode splitter for when method sizes
exceed the limit allowed by the JVM (embedded from another project).

asmble.compile.jvm.InsnReworker#addEagerLocalInitializers - Backwards navigation up the instruction list to make
sure that a local is set before it is get.

asmble.compile.jvm.InsnReworker#injectNeededStackVars - Inject instructions at certain places to make sure we have
certain items on the stack when we need them.

asmble.run.jvm.interpret.Interpreter - Full WASM interpreter in a few hundred lines of Kotlin.

Compilation Details

Asmble does its best to compile WASM ops to JVM bytecodes with minimal overhead. Below are some details on how each part
is done. Every module is represented as a single class. This section assumes familiarity with WebAssembly concepts.

Constructors

Asmble creates different constructors based on the memory requirements. Each constructor created contains the imports as
parameters (see imports below)

If the module does not define memory, a single constructor is created that accepts all other imports. If the module does
define memory, two constructors are created: one accepting a memory instance, and an overload that instead accepts an
integer value for max memory that is used to create the memory instance before sending to the first one. If the maximum
memory is given for the module, a third constructor is created without any memory parameters and just calls the max
memory overload w/ the given max memory value. All three of course have other imports as the rest of the parameters.

After all other constructor duties (described in sections below), the module’s start function is called if present.

Memory

Memory is built or accepted in the constructor and is stored in a field. The current implementation uses a ByteBuffer.
Since ByteBuffers are not dynamically growable, the max memory is an absolute max even though there is a limit which
is adjusted on grow_memory. Any data for the memory is set in the constructor.

Table

In the WebAssembly MVP a table is just a set of function pointers. This is stored in a field as an array of
MethodHandle instances. Any elements for the table are set in the constructor.

Globals

Globals are stored as fields on the class. A non-import global is simply a field that is final if not mutable. An import
global is a MethodHandle to the getter and a MethodHandle to the setter if mutable. Any values for the globals are
set in the constructor.

Imports

The constructor accepts all imports as params. Memory is imported via a ByteBuffer param, then function
imports as MethodHandle params, then global imports as MethodHandle params (one for getter and another for setter if
mutable), then a MethodHandle array param for an imported table. All of these values are set as fields in the
constructor.

Exports

Exports are exported as public methods of the class. The export names are mangled to conform to Java identifier
requirements. Function exports are as is whereas memory, global, and table exports have the name capitalized and are
then prefixed with “get” to match Java getter conventions.

Exports are always separate methods instead of just changing the name of an existing method or field. This encapsulation
allows things like many exports for a single item.

Types

Control Flow Operations

Operations such as unreachable (which throws) behave mostly as expected. Branching and looping are handled with jumps.
The problem that occurs with jumping is that WebAssembly does not require compiler writers to clean up their own stack.
Therefore, if the WASM ops have extra stack values, we pop it before jumping which has performance implications but not
big ones. For most sane compilers, the stack will be managed stringently and leftover stack items will not be present.

Luckily, br_table jumps translate literally to JVM table switches which makes them very fast. There is a special set
of code for handling really large tables (because of Java’s method limit) but this is unlikely to affect most in
practice.

Call Operations

Normal call operations do different things depending upon whether it is an import or not. If it is an import, the
MethodHandle is retrieved from a field and called via invokeExact. Otherwise, a normal invokevirtual is done to
call the local method.

A call_indirect is done via invokedynamic on the JVM. Specifically, invokedynamic specifies a synthetic bootstrap
method that we create. It does a one-time call on that bootstrap method to get a MethodHandle that can be called in
the future. We wouldn’t normally have to use invokedynamic because we could use the index to reference a
MethodHandle in the array field. However, in WebAssembly, that index is after the parameters of the call and the
stack manipulation we would have to do would be far too expensive.

So we need a MethodHandle that takes the params of the target method, and then the index, to make the call. But we
also need “this” because it is expected at some point in the future that the table field could be changed underneath and
we don’t want that field reference to be cached via the one-time bootstrap call. We do this with a synthetic bootstrap
method which uses some MethodHandle trickery to manipulate it the way we want. This makes indirect calls very fast,
especially on successive invocations.

Parametric Operations

A drop translates literally to a pop. A select translates to a conditional swap, then a pop.

Variable Access

Local variable access translates fairly easily because WebAssembly and the JVM treat the concept of parameters as the
initial locals similarly. Granted the JVM form has “this” at slot 0. Also, WebAssembly doesn’t treat 64-bit vars as 2
slots like the JVM, so some simple math is done like it is with the stack.

WebAssembly requires all locals the assume they are 0 whereas the JVM requires locals be set before use. An algorithm in
Asmble makes sure that locals are set to 0 before they are fetched in any situation where they weren’t explicitly set
first.

The WebAssembly spec requires a runtime check of overflow during trunc calls. This is enabled by default in Asmble. It
defers to an internal synthetic method that does the overflow check. This can be programmatically disabled for better
performance.

Stack

Asmble maintains knowledge of types on the stack during compilation and fails compilation for any invalid stack items.
This includes the somewhat complicated logic concerning unreachable code.

In several cases, Asmble needs something on the stack that WebAssembly doesn’t, such as “this” before the value of a
putfield call when setting a non-import global. In order to facilitate this, Asmble does a preprocessing of the
instructions. It builds the stack diffs and injects the needed items (e.g. a reference to the memory class for a load)
at the right place in the instruction list to make sure they are present when needed.

As an unintended side effect of this kind of logic, it turns out that Asmble never needs local variables beyond what
WebAssembly specifies. No temp variables or anything. It could be argued however that the use of temp locals might make
some of the compilation logic less complicated and could even improve runtime performance in places where we overuse the
stack (e.g. some places where we do a swap).

Caveats

Below are some performance and implementation quirks where there is a bit of an impedance mismatch between WebAssembly
and the JVM:

WebAssembly has a nice data section for byte arrays whereas the JVM does not. Right now we use a single-byte-char
string constant (i.e. ISO-8859 charset). This saves class file size, but this means we call String::getBytes on
init to load bytes from the string constant. Due to the JVM using an unsigned 16-bit int as the string constant
length, the maximum byte length is 65536. Since the string constants are stored as UTF-8 constants, they can be up to
four bytes a character. Therefore, we populate memory in data chunks no larger than 16300 (nice round number to make
sure that even in the worse case of 4 bytes per char in UTF-8 view, we’re still under the max).

The JVM makes no guarantees about trailing bits being preserved on NaN floating point representations like WebAssembly
does. This causes some mismatch on WebAssembly tests depending on how the JVM “feels” (I haven’t dug into why some
bit patterns stay and some don’t when NaNs are passed through methods).

The JVM requires strict stack management where the compiler writer is expected to pop off what he doesn’t use before
performing unconditional jumps. WebAssembly requires the runtime to discard unused stack items before unconditional
jump so we have to handle this. This can cause performance issues because essentially we do a “pop-before-jump” which
pops all unneeded stack values before jumping. If the target of the jump expects a fresh item on the stack (i.e. a
typed block) then it gets worse because we have to pop what we don’t need except for the last stack value which
leads to a swap-pop-and-swap. Hopefully in real world use, tools that compile to WebAssembly don’t have a bunch of
these cases. If they do, we may need to look into spilling to temporary local vars.

Both memory and tables have “max capacity” and “initial capacity”. While memory uses a ByteBuffer which has these
concepts (i.e. “capacity” and “limit”), tables use an array which only has the “initial capacity”. This means that
tests that check for max capacity on imports at link time do not fail because we don’t store max capacity for a table.
This is not a real problem for the MVP since the table cannot be grown. But once it can, we may need to consider
bringing another int along with us for table max capacity (or at least make it an option).

WebAssembly has a concept of “unset max capacity” which means there can theoretically be an infinite capacity memory
instance. ByteBuffers do not support this, but care is taken to allow link time and runtime max memory setting to
give the caller freedom.

WebAssembly requires some trunc calls to do overflow checks, whereas the JVM does not. So for example, WebAssembly
has i32.trunc_s/f32 which would usually be a simple f2i JVM instruction, but we have to do an overflow check that
the JVM does not do. We do this via a private static synthetic method in the module. There is too much going on to
inline it in the method and if several functions need it, it can become hot and JIT’d. This may be an argument for a
more global set of runtime helpers, but we aim to be runtime free. Care was taken to allow the overflow checks to be
turned off programmatically.

WebAssembly allows unsigned 32 bit int memory indices. ByteBuffer only has signed which means the value can
overflow. And in order to support even larger sets of memory, WebAssembly supports constant offsets which are added
to the runtime indices. Asmble will eagerly fail compilation if an offset is out of range. But at runtime we don’t
check by default and the overflow can wrap around and access wrong memory. There is an option to do the overflow check
when added to the offset which is disabled by default. Other than this there is nothing we can do easily.

FAQ

Why?

I like writing compilers and I needed a sufficiently large project to learn Kotlin really well to make a reasonable
judgement on it. I also wanted to become familiar w/ WebAssembly. I don’t really have a business interest for this and
therefore I cannot promise it will forever be maintained.

Will it work on Android?

I have not investigated. But I do use invokedynamic and MethodHandle so it would need to be a modern version of
Android. I assume, then, that both runtime and compile-time code might run there. Experiment feedback welcome.

What about JVM to WASM?

This is not an immediate goal of this project, at least not until the WASM GC proposal has been accepted. In the
meantime, there is https://github.com/konsoletyper/teavm

So I can compile something in C via Emscripten and have it run on the JVM with this?

Yes, but work is required. WebAssembly is lacking any kind of standard library. So Emscripten will either embed it or
import it from the platform (not sure which/where, I haven’t investigated). It might be a worthwhile project to build a
libc-of-sorts as Emscripten knows it for the JVM. Granted it is probably not the most logical approach to run C on the
JVM compared with direct LLVM-to-JVM work.

Debugging?

Not yet, once source maps get standardized I may revisit.

TODO

Add “dump” that basically goes from WebAssembly to “javap” like output so details are clear

Expose the advanced compilation options

Add “link” command that will build an entire JAR out of several WebAssembly files and glue code between them

Annotations to make it clear what imports are expected

Compile some parts to JS and native with Kotlin

Add javax.script support (which can give things like a free repl w/ jrunscript)